1. Field of the Invention
The present invention relates to a coil spring used as a torsion spring for a torsion damper, a valve spring, a return spring, a suspension spring, a die spring, or the like.
2. Description of Related Art
Among torsion springs and the like that are used for, for example, clutch disks, there is one that has a spring wire wound in a coil shape and having flat faces formed on a outer shape of a circular cross section thereof, the flat faces serving as contact faces.
When load is applied to the coil spring to press the coil shape into a tightly compressed state or a locked state, the flat faces of adjacent coil portions come into contact with each other to stably receive the load and suppress slippage in a coil diameter direction.
In general, stress on an inner diameter side (coil inner diameter section) of the coil shape of the spring wire of the coil spring is higher than that on an outer diameter side (coil outer diameter section) of the same. In addition, forming the flat faces results in changing a sectional flattening and affecting, together with the stress deviation, a stress distribution state of the spring wire in a sectional circumferential direction.
The spring wire that is provided with the flat faces may reduce a flattening of the sectional shape thereof, to shorten a solid length in a coil axis direction when the flat faces are brought into contact with each other. This configuration is advantageous when designing a spring having a long stroke and low rigidity.
Accordingly, determining a sectional flattening for a spring wire is important in realizing a coil spring having a good stress distribution and better design.
A conventionally suggested flattening is 0.85 to 0.98.
This flattening range, however, has limits when designing a coil spring better than a coil spring whose sectional outer shape is circular, and therefore, further improvements are required.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H06-300065